The present invention relates generally to cables, and more particularly to compositions suitable for semiconductive jackets especially for medium and high voltage power cables and cables jacketed therewith.
Electric power cables for medium and high voltages typically include a core electrical conductor, an overlaying semiconductive shield, an insulation layer formed over the semiconductive shield, an outermost insulation shield, and some type of metallic component. The metallic component may include, for example, a lead sheath, a longitudinally applied corrugated copper tape with overlapped seam, or helically applied wires, tapes, or flat strips. U.S. Pat. No. 5,281,757 assigned to the current assignee, and U.S. Pat. No. 5,246,783, the contents of both of which are herein incorporated by reference, disclose examples of electric power cables and methods of making the same.
Electric power cables for medium and high voltage applications also typically include an overall extruded plastic jacket which physically protects the cable thereby extending the useful life of the cable. The afore-described overall jacket may be insulating or semiconducting. If the overall jacket is insulating, it may overlay or encapsulate the metallic component of the cable as discussed in the September/October 1995 Vol. 11, No. 5, IEEE Electrical Insulation Magazine article, entitled, Insulating and Semiconductive Jackets for Medium and High Voltage Underground Power Cable Applications, the contents of which are herein incorporated by reference.
According to the National Electrical Safety Code, power cables employing insulating jackets must be grounded every 0.125 to 0.25 mile depending on the application, or at every splice for cable in duct (at every manhole). Such grounding reduces or eliminates the losses in a cable system. Furthermore, as the neutral to ground voltage may be very high, such grounding is also required for safety purposes.
In contrast to insulating jackets, semiconductive jackets are advantageously grounded throughout the length of the cable and therefore do not need periodic grounding previously described. Accordingly, semiconductive jackets are only grounded at the transformer and at the termination.
Although semiconductive jackets are advantageous for the foregoing reasons, they are not widely employed in the power cable industry. Prior art semiconductive jacket materials were usually not developed for jacketing applications, and as such, often do not meet performance criteria for long-life cable protection.
The Insulated Cable Engineers Association (ICEA) specifies in ICEA S-94-649-1997, xe2x80x9cSemiconducting Jacket Type 1xe2x80x9d, mechanical properties for semiconductive electrical cable jackets and references American Society for Testing and Materials (ASTM) test methods to test materials suitable for these applications.
Prior art semiconductive jackets, even if they do meet performance criteria for long-life cable protection, are often cost prohibitive for widespread industry employment. This high cost is primarily due to the high weight percentage of conductive additive necessary in the jacket material to make the jacket semiconductive. Typically this weight percentage is greater than 15 to 30 weight percent to achieve the required conductivity or volume resistivity for the jacket. See, for example, U.S. Pat. No. 3,735,025, the contents of which are herein incorporated by reference, which discloses an electric cable jacketed with a thermoplastic semiconducting composition comprising chlorinated polyethylene, ethylene ethyl acrylate, and 30 to 75 or 40 to 60 parts by weight of semiconducting carbon black.
Prior art polymer compounds used in the role of a semiconductive jackets are normally thermoplastic and get their conductivity by use of a large weight percentage of a conductive filler material, usually conductive grades of carbon black, to incur a high level of conductivity (or low level of resistivity), to the compound. The National Electrical Safety Code (Section 354D2-c) requires a radial resistivity of the semiconducting jacket to be not more than 100 xcexa9xc2x7m and shall remain essentially stable in service. Prior art compositions required loadings of conductive filler material of at least about 15% to 60% by weight to achieve this criteria. These high levels of conductive filler material inherently add significantly to the cost of such compositions, inhibit the ease of extrusion of the jacketing composition, and decrease the mechanical flexibility of the resultant cable.
Percolation theory is relatively successful in modeling the general conductivity characteristics of conducting polymer composite (CPC) materials by predicting the convergence of conducting particles to distances at which the transfer of charge carriers between them becomes probable. The percolation threshold (pc), which is the level at which a minor phase material is just sufficiently incorporated volumetrically into a major phase material resulting in both phases being co-continuous, that is, the lowest concentration of conducting particles needed to form continuous conducting chains when incorporated into another material, can be determined from the experimentally determined dependence of conductivity of the CPC material on the filler concentration. For a general discussion on percolation theory, see the October 1973 Vol. 45, No. 4, Review of Modem Physics article, entitled, Percolation and Conduction, the contents of which are herein incorporated by reference. Much work has been done on determining the parameters influencing the percolation threshold with regard to conductive filler material. See for example, Models Proposed to Explain the Electrical Conductivity of Mixtures Made of Conductive and Insulating Materials, 1993 Journal of Materials Science, Vol. 28; Resistivity of Filled Electrically Conductive Crosslinked Polyethylene, 1984 Journal of Applied Polymer Science, Vol. 29; and Electron Transport Processes in Conductor-Filled Polymers, 1983 Polymer Engineering and Science Vol. 23, No. 1; the contents of each of which are herein incorporated by reference. See also, Multiple Percolation in Conducting Polymer Blends, 1993 Macromolecules Vol. 26, which discusses xe2x80x9cdouble percolationxe2x80x9d, the contents of which are also herein incorporated by reference.
Attempts for the reduction of conductive filler content in CPC materials have been reported for polyethylene/polystyrene and for polypropylene/polyamide, both employing carbon black as the conductive filler. See Design of Electrical Conductive Composites: Key role of the Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends, 1995 Macromolecules, Vol. 28 No. 5 and Conductive Polymer Blends with Low Carbon Black Loading: Polypropylene/Polyamide, 1996 Polymer Engineering and Science, Vol. 36, No. 10, the contents of both of which are herein incorporated by reference.
However, none of the prior art concerned with minimizing the conductive filler content has addressed materials suitable for use as a semiconductive jacket material for cables which must meet not only the electrical requirements, but also stringent mechanical requirements as discussed heretofore.
What is needed, and apparently lacking in the art is a semiconductive jacket material which has a significant reduction of conductive filler material, thereby decreasing the cost of the material and the processing by increasing the ease of extrusion and mechanical flexibility of the jacketed cable, while maintaining industry performance criteria for resistivity and mechanical properties.
The present invention provides a conductive polymer composite (CPC) material for semiconductive jackets for cables which has a significant reduction in conductive filler content while maintaining the required conductivity and mechanical properties specified by industry by selecting materials and processing approaches to reduce the percolation threshold of the conductive filler in the composite, while balancing the material selection with the industry required mechanical properties of the semiconductive jacket.
The present inventive semiconductive jackets for cables share certain attributes with U.S. application Ser. No. 09/113,963, entitled, Conductive Polymer Composite Materials And Methods of Making Same, (presently pending) filed on an even date herewith by the same applicant, the contents of which are herein incorporated by reference. That is, the semiconductive jacket materials of the present invention are based on immiscible polymer blends wherein the immiscibility is exploited to create semiconductive compounds with low content conductive filler through a multiple percolation approach to network formation. The conductive filler material content can be reduced to about 6% by weight of the total composite or less, depending on the conductive filler material itself and the selection of major and minor phase materials, without a corresponding loss in the conductivity performance of the compound. Correspondingly, the rheology of the melt phase of the inventive material will more closely follow an unfilled system due to the reduction of the reinforcing conductive filler content thereby increasing the ease of processing the material.
Semiconductive jackets for power cables must have a conductive network throughout the material. The physics of network formation of a minor second phase material in a differing major phase is effectively described by percolation theory as discussed heretofore. The xe2x80x9cpercolation thresholdxe2x80x9d (pc) is the level at which a minor phase material is just sufficiently incorporated volumetrically into a major phase material resulting in both phases being co-continuous, that is, the lowest concentration of conducting particles needed to form continuous conducting chains when incorporated into another material. A minor second phase material in the form of nonassociating spheres, when dispersed in a major phase material, must often be in excess of approximately 16% by volume to generate an infinite network. This 16 volume % threshold, which is exemplary for spheres, is dependent on the geometry of the conductive filler particles, (i.e. the surface area to volume ratio of the particle) and may vary with the type of filler. The addition of a single dispersion of conductor filler particles to a single major phase is termed xe2x80x9csingle percolationxe2x80x9d. It has been found that by altering the morphology of the minor/major phase a significant reduction in percolation threshold can be realized. The present invention exploits these aspects of percolation theory in developing very low conductive filler content semiconductive jacket materials for cables.
In accordance with the present invention, a method requiring an immiscible blend of at least two polymers that phase separate into two continuous morphologies is employed. By requiring the conductive filler to reside in the minor polymer phase, the concentration of conductive filler can be concentrated above the percolation threshold required to generate a continuous conductive network in the minor polymer phase while the total concentration of conductive filler in the volume of the combined polymers is far below the threshold if the filler was dispersed uniformly throughout both phases. In addition, since the minor polymer phase is co-continuous within the major polymer phase, the concentration is conductive. This approach employs multiple percolation due to the two levels of percolation that are required: percolation of conductive dispersion in a minor phase and percolation of a minor phase in a major phase.
In a binary mixture of a semicrystalline polymer and a conductive filler, the filler particles are rejected from the crystalline regions into the amorphous regions upon recrystallization, which accordingly decreases the percolation threshold. Similarly, using a polymer blend with immiscible polymers which results in dual phases as the matrix in CPC materials promotes phase inhomogeneities and lowers the percolation threshold. The conductive filler is heterogeneously distributed within the polymers in this latter example. In one alternative of this approach, either one of the two polymer phases is continuous and conductive filler particles are localized in the continuous phase. In a second alternative, the two phases are co-continuous and the filler is preferably in the minor phase or at the interface.
The present invention concentrates primarily on two aspects of percolation phenomenon: the interaction of the conductive dispersion in the minor phase, and the interaction of the minor phase with the major phase. Further, the foregoing approach as disclosed in the afore-referenced U.S. application Ser. No. 09/113,963, entitled, Conductive Polymer Composite Materials and Methods of Making Same now U.S. Pat. No. 6,277,303 may be employed and has been optimized and balanced for semiconductive jacket applications.
In accordance with one aspect of the present invention, a semiconductive jacket material for jacketing a cable comprises: a minor phase material comprising a semicrystalline polymer; a conductive filler material dispersed in said minor phase material in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network in said minor phase material; and a major phase material, said major phase material being a polymer which when mixed with said minor phase material will not engage in electrostatic interactions that promote miscibility, said major phase material having said minor phase material dispersed therein in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network in said major phase material, forming a semiconductive jacket material of a ternary composite having distinct co-continuous phases.
In accordance with another aspect of the present invention, the ternary composite has a volume resistivity of about xe2x89xa6100 xcexa9xc2x7m, an unaged tensile strength of at least about 1200 psi, a tensile strength of at least about 75% of said unaged tensile strength after aging in an air oven at 100xc2x0 C. for 48 hours, an aged and unaged elongation at break of at least about 100%, a heat distortion at 90xc2x0 C. of at least about xe2x88x9225%, and a brittleness temperature of about xe2x89xa6xe2x88x9210xc2x0 C.
In accordance with another aspect of the present invention, the conductive filler material comprises about xe2x89xa66 percent by weight of total conducting polymer composite weight.
In accordance with yet another aspect of the present invention, the semiconductive jacket material further comprises a second major phase material, wherein said ternary composite is dispersed in an amount sufficient for said ternary composite to be continuous within said second major phase material, said second major phase material being selected from a group of polymers which when mixed with said ternary composite will not engage in electrostatic interactions that promote miscibility with said minor phase material or said major phase material, forming a semiconductive jacket material of a quaternary composite having distinct co-continuous phases.
In accordance with a further aspect of the present invention, a method of producing a semiconductive jacket material for jacketing a cable comprises: mixing a semicrystalline polymer having a melting temperature in a mixer, said mixer preheated to above the melting temperature of said semicrystalline polymer; adding a conductive filler material to said semicrystalline polymer in said mixer in an amount xe2x89xa7 an amount required to generate a continuous conductive network in said semicrystalline polymer; mixing said conductive filler material and said semicrystalline for a time and at a speed sufficient to insure a uniform distribution of said conductive filler in said semicrystalline polymer, thereby forming a binary composite; and mixing a major phase material having a melting temperature with said binary composite in said mixer preheated to above the melting temperature of said major phase material, for a time and at a speed sufficient to insure a uniform distribution of said binary composite in said major phase material, such that a weight ratio of said binary composite to said major phase material is sufficient for said binary composite to be xe2x89xa7 an amount required to generate a continuous conductive network in said major phase material, said major phase material being selected from a group of polymers which when mixed with said binary composite will not engage in electrostatic interactions that promote miscibility, such that a semiconductive jacket material of a ternary composite with distinct co-continuous phases is formed.
In accordance with yet a further aspect of the present invention, a method of producing a semiconductive jacket material for jacketing a cable comprises: mixing a semicrystalline minor phase polymer material with a conductive filler material, the conductive filler material being in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network within the minor phase polymer material, thereby forming a binary composite; mixing the binary composite with a major phase polymer material to form a semiconductive jacket material of a ternary composite having distinct phases; and annealing the ternary composite to coarsen the morphology and thereby further increase conductivity of the jacket material, said major phase polymer material being selected from a group of polymers which when mixed with said binary composite will not engage in electrostatic interactions that promote miscibility, such that a semiconductive ternary composite with distinct co-continuous phases is formed.
In accordance with yet a further aspect of the present invention, a method of producing a semiconductive jacket material for jacketing a cable comprises: mixing a semicrystalline minor phase polymer material having a melting temperature with a conductive filler material, the conductive filler material being in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network within the minor phase polymer material, thereby forming a binary composite; annealing the binary composite; and mixing the binary composite with a major phase material at a temperature below the melting temperature of the binary composite, said major phase polymer material being selected from a group of polymers which when mixed with said binary composite will not engage in electrostatic interactions that promote miscibility, thereby forming a semiconductive jacket material of a ternary composite having distinct co-continuous phases.
Still further in accordance with the present invention, a method of producing a semiconductive jacket material for jacketing a cable comprises: mixing a semicrystalline minor phase polymer material with a conductive filler material, the conductive filler material being in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network within the minor phase polymer material, thereby forming a binary composite; mixing the binary composite with a major phase polymer material to form a ternary composite; mixing the ternary composite with a second major phase polymer material to form a semiconductive jacket material of a quaternary composite having distinct phases; and annealing the quaternary composite to coarsen the morphology and thereby further increase the conductivity of the jacket material, said major phase polymer material being selected from a group of polymers which when mixed with said binary composite will not engage in electrostatic interactions that promote miscibility, such that a semiconductive ternary composite with distinct co-continuous phases is formed.
In further accordance with the present invention, a cable comprises at least one transmission medium and a semiconductive jacket surrounding said transmission medium, said semiconductive jacket comprising: a minor phase material comprising a semicrystalline polymer; a conductive filler material dispersed in said minor phase material in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network in said minor phase material; and a major phase material, said major phase material being a polymer which when mixed with said minor phase material will not engage in electrostatic interactions that promote miscibility, said major phase material having said minor phase material dispersed therein in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network in said major phase material, forming a semiconductive jacket material of a ternary composite having distinct co-continuous phases.
In general, the superior results of the present invention may be achieved by allowing the conductive filler material to reside in a minor phase of the immiscible blend; the minor phase being a semicrystalline polymer having a relatively high crystallinity, such as between about 20% and about 80%, and preferably about xe2x89xa770%, thereby causing the conductive filler aggregates to concentrate in amorphous regions of the minor phase or at the interface of the continuous minor and major phases. Annealing processes of the composite at different points in the mixing process or modifying the morphology of the minor phase can further increase the crystalline phase or further coarsen the morphology of the blend and thereby improve the conductive network.
In accordance with the present invention, in order that a favorable phase morphology, that is, phase separation, develops between minor and major phase materials, the minor and major phase materials must be such that when mixed, the minor and major phase polymeric materials do not engage in electrostatic interactions that promote miscibility resulting in a negative enthalpy of mixing. Thus, hydrogen bonding does not occur between any of the phases and there is phase separation between all of the phases.
An advantage of the present invention includes the reduction of conductive filler material content in a semiconductive cable jacket to less than about 6 weight percent of total composite weight without a corresponding loss in the conductivity performance of the jacket.
Yet another advantage of the present invention is the ability to produce a semiconductive cable jacket which satisfies the ICEA S-94-649-1997 xe2x80x9cSemiconducting Jacket Type 1xe2x80x9d specification requirements.
Yet another advantage is the cost reduction due to the reduced conductive filler content and ease of processing over conventional semiconducting jackets.